Some polymers are known to degrade by hydrolysis in the presence of water and thereby decompose to smaller chemical units. Some of these polymers are also biodegradable, such as polylactide, polyglycolide, poly-.epsilon.-caprolactone and polydioxaneone (2-keto-1,4-dioxane).
Polymers such as polylactide, polyglycolide, and poly(2-keto-1,4-dioxane) can be referred to generally as polydioxanediones, because each is prepared by polymerization of a dioxanedione-based monomer. As used herein, except as specifically noted otherwise, dioxaneone refers to compounds having a dioxane ring with at least one carbonyl oxygen pendant from the dioxane ring. The remaining three carbon atoms in the dioxane ring may have various constituents pendant therefrom. Although the term dioxaneone, which is also sometimes written as dioxanone, is often used in a specific sense to refer to 2-keto-1,4-dioxane, dioxaneone is used herein in a general sense as discussed below, unless otherwise specifically indicated by the general formula: ##STR1## where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 can be any of a variety of constituents and where Z can be one or more constituents covalently bonded to the associated tetravalent carbon atom in the dioxane ring. When all of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are hydrogen and Z is two hydrogen constituents, then the compound is 2-keto-1,4-dioxane.
Dioxaneones such as lactide and glycolide, in which Z is a carbonyl oxygen, may be more specifically referred to as dioxanediones since they each have two carbonyl oxygens pendant from the dioxane ring. Dioxanediones are cyclic diesters that may be represented by the general formula: ##STR2## Where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 can be any of a variety of constituents. When R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are all hydrogen, then the compound is glycolide which is also referred to as 1,4-dioxane-2,5-dione. Although the term dioxanedione is sometimes used to refer specifically to glycolide, the term as used herein is always employed in the general sense to indicate a class of compounds as indicated by the generic formula above, except as otherwise noted herein. When R.sub.1 and R.sub.3 are methyl and R.sub.2 and R.sub.4 are hydrogen the compound is lactide, which may be also referred to as 3,6-dimethyl-1,4-dioxane-2,5-dione. A polydioxaneone having one or more repeating units representative of a dioxanedione monomer may be more specifically referred to as a polydioxanedione. When a dioxaneone contains one or more asymmetrical carbon atoms, such as is the case with lactide, then that particular dioxaneone can exist as various optical isomers. For example, lactide can exist as two optically active isomers, D-lactide and L-lactide, or as the optically inactive isomer meso-lactide. D-lactide and L-lactide can also be present in equal quantities to form an optically inactive mixture known as racemic-lactide. Both meso-lactide and racemic-lactide are often designated as simply D,L-lactide.
Most reported polydioxaneones are homopolymers, although some attempts have also been reported for the preparation of copolymers between two different dioxaneone monomers. For example, U.S. Pat. No. 2,703,316 by Schneider, issued Mar. 1, 1955, shows two examples of copolymers between different dioxanediones. One copolymer was made from equal parts of glycolide and lactide that is described as being hot pressed into a film. Another copolymer was made from 12 parts of lactide and 2 parts of tetramethylglycolide (3,3,6,6-tetramethyl-1,4-dioxane-2,5-dione). No properties of the latter copolymer were reported and no indication was given that the copolymer could be incorporated into useful product forms.
Due to the expense and difficulty in preparing polydioxaneones, their use has been largely confined to high-value medical applications where bioabsorbable materials are required. Most reported medical applications involve use of the polymers internally in the human body, such as for use as sutures, prosthetic devices, and carriers for the controlled release of pharmaceuticals. Even in the medical field, however, few copolymers between different dioxaneones have been reported, with the exception of copolymers between lactide and glycolide and various copolymers between different optical isomers of lactide, and copolymers of 2-keto-1,4-dioxane with glycolide or lactide. Medical applications involve relatively predictable and constant environmental conditions to which the polymers are subjected during use, i.e., the human body. Therefore, the need to manipulate or modify the properties of polymers used in such medical applications has not been great.
One reference, U.S. Pat. No. 3,636,956 by Schneider, issued Jan. 25, 1972, discloses making filaments that could be useful as sutures from various copolymers using lactide. One example shows polymerization of 44.2 parts of L-lactide and 5.8 parts of the cyclic ester of .alpha.-hydroxybutyric acid (3,6-diethyl-1,4-dioxane-2,5-dione). Another example shows 45 parts of L-lactide copolymerized with 5 parts of the cyclic ester of .alpha.-hydroxyheptanoic acid (3,6-dipentyl-1,4-dioxane-2,5-dione). Both copolymers were spun through a 35 mil spinneret and then drawn to form filaments.
Due to the current lack of available landfill space and the concern over environmental contamination, a need exists for polymers that can be used to make degradable plastic products for mass market applications. Little attention has been given, however, to the use of polydioxaneones for mass-market consumer applications. Such potential mass-market applications include, for example, foam materials, molded products, pellet materials, adhesives, laminates, nonwoven materials and films.
Some attempts have been made to make degradable products for mass market applications. U.S. Pat. No. 5,076,983 by Loomis et al., issued Dec. 31, 1991, discusses preparation of film materials from certain "polyhydroxy acids," including polymers made from lactide monomers. The film materials of Loomis et al. require the inclusion of external plasticizers.
The use of external plasticizers as required by U.S. Pat. No. 5,076,983, however, may add to the expense and complexity of preparing the materials. As discussed in U.S. Pat. No. 5,180,765 by Sinclair, issued Jan. 19, 1993, such external plasticizers must be intimately dispersed within the polymer composition. Also, some such external plasticizers tend to migrate and segregate from the polymer during processing at elevated temperatures further complicating the manufacture of many useful product forms.
Polymers of the same chemical structure as a polydioxanedione can be prepared by directly polymerizing .alpha.-hydroxycarboxylic acids in a condensation polymerization reaction. Such polymers, however, suffer from a low molecular weight. For example, Fukuzaki et al., Low-Molecular-Weight Copolymers Composed of L-lactic Acid and Various DL-Hydroxy Acids as Biodegradable Carriers, Makromol. Chem. 190, 2571-2577 (1989), discloses 70/30, molar ratio, of condensation copolymers made from L-lactic acid/D,L-lactic acid, L-lactic acid/D,L-.alpha.-hydroxybutyric acid, L-lactic acid/D,L-.alpha.-hydroxyisovaleric acid, and L-lactic acid/D,L-.alpha.-hydroxyisocaproic acid. As with other reported condensation polymers prepared from .alpha.-hydroxycarboxylic acids, however, those polymers were all of low molecular weight, on the order of a few thousand. Such low molecular weights may be adequate for some applications, such as some drug delivery applications as suggested by Fukuzaki et al., but have limited potential for mass-market product applications.
Higher molecular weight polymers have been produced by ring-opening polymerization of dioxanedione monomers. Dioxanediones used as monomers to produce higher molecular weight polymers have traditionally been made from low molecular weight poly-.alpha.-hydroxycarboxylic acids by a depolymerization reaction often referred to as "backbiting." These low molecular weight polymers are often referred to as oligomers. The backbiting process is slow and expensive, contributing to the lack of interest in attempting to develop low-cost consumer products for mass-market applications using polydioxanedione polymers.